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Ultraviolet Radiations

1801

Johann Wilhelm Ritter

(generated image for illustration only)

In 1801, Johann Wilhelm Ritter observed that invisible rays beyond the violet end of the solar spectrum darkened silver chloride-soaked paper more quickly than violet light itself. This demonstrated the existence of a form of light beyond the visible spectrum, which he termed “oxidizing rays” to contrast with “heat rays” (infrared) discovered the previous year.

Johann Wilhelm Ritter’s experiment was a direct follow-up to William Herschel’s discovery of infrared radiation in 1800. Herschel had used a prism to split sunlight and placed thermometers beyond the red end of the spectrum, noting a temperature increase. Ritter, intrigued by this, decided to test the other end of the spectrum. He knew that silver chloride decomposed in the presence of light, and that this effect was more pronounced in violet light. He exposed strips of paper soaked in silver chloride to the solar spectrum produced by a prism. As expected, the paper darkened in the visible spectrum, with the effect increasing from red to violet. Crucially, he placed a strip just beyond the violet end, in a region where no visible light was present, and found that this strip darkened even more rapidly than the one in the violet light. This confirmed the existence of radiation with chemical activity stronger than visible light, located beyond the violet. His initial name, “oxidizing rays,” highlighted its chemical properties, which was later replaced by “chemical rays” and eventually “ultraviolet rays.” This discovery fundamentally expanded the known electromagnetic spectrum, proving that human vision perceives only a small fraction of it.

Chemical Recycling, Chemistry, Electromagnetism, Optics, Photonics, Physics, Spectroscopy

UNESCO Nomenclature: 2210

– Optics

Type

Scientific Discovery

Disruption

Foundational

Usage

Widespread Use

Precursors

Isaac Newton’s work on the visible spectrum using prisms
William Herschel’s discovery of infrared radiation (1800)
Carl Wilhelm Scheele’s observations on the darkening of silver salts by light

Applications

spectroscopy
uv photography
medical imaging
sterilization techniques
astronomical observation

Patents:

Potential Innovations Ideas

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Related to: ultraviolet discovery, Johann Ritter, silver chloride, electromagnetic spectrum, spectroscopy, actinic rays, chemical rays, optics, physics history, light.

Historical Context

Alessandro Volta's laboratory demonstrating electromotive force with early electrical apparatus.

Definition of Electromotive Force (EMF)

Electromotive force (\(\mathcal{E}\)) is the work done per unit of electric charge by a non-electrical source, such as a battery or generator. Despite its name, it is not a mechanical force but an electric potential, measured in volts. It represents the energy converted from another form (chemical, mechanical) into electrical energy as a charge traverses the source.

Voltaic pile demonstrating electrochemical reaction with zinc and copper electrodes.

Electrochemical Reaction in the Voltaic Pile

The electric current in a Voltaic pile is produced by a redox reaction. At the zinc anode, zinc metal is oxidized, releasing two electrons per atom (\(Zn \rightarrow Zn^{2+} + 2e^{-}\)). These electrons travel through the external circuit to the copper cathode. There, hydrogen ions from the aqueous electrolyte are reduced, forming hydrogen gas (\(2H^{+} + 2e^{-} \rightarrow H_2\)).

Absolute Humidity

Absolute humidity (\(d_v\)) is the total mass of water vapor (\(m_{H_2O}\)) present in a given volume of air (\(V_{air}\)). It is expressed as grams of water vapor per cubic meter of air (\(g/m^3\)). The formula is \(d_v = \frac{m_{H_2O}}{V_{air}}\). Unlike relative humidity, it is not dependent on the air's temperature, but it is affected by changes in air pressure or volume.

Ultraviolet Radiations

Glass apparatus demonstrating Charles's Law in a vintage laboratory setting.

Charles’s Law (Vol-Temp Law)

Also known as the volume-temperature Law or law of volumes, Charles's Law states that for a fixed mass of an ideal gas at constant pressure, the volume it occupies is directly proportional to its absolute temperature. This is expressed as \(V \propto T\) or \(\frac{V_1}{T_1} = \frac{V_2}{T_2}\). It explains the tendency of gases to expand when heated under isobaric (constant pressure) conditions.

19th-century laboratory with scientists studying atomic structure and chemical reactions.

Modern Atomic Theory

Atomic theory posits that all matter is composed of discrete units called atoms. An element consists of atoms with a specific number of protons in their nucleus. While once considered indivisible, atoms are now known to comprise subatomic particles: protons, neutrons, and electrons. Chemical reactions involve the rearrangement of these atoms, which are conserved during the process.

Avogadro’s Law

Avogadro's law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules. This establishes a direct proportionality between the volume of a gas and the amount of substance (number of moles). The mathematical relationship is expressed as \(V \propto n\) or, more commonly, as \(V/n = k\), where k is a constant.

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Galvanic Corrosion

Galvanic corrosion is an electrochemical process where one metal corrodes preferentially when in electrical contact with another in the presence of an electrolyte. This occurs because the dissimilar metals create a bimetallic couple, with the less noble (more active) metal acting as the anode and corroding, while the more noble metal acts as the cathode.

Voltaic pile demonstrating early electrochemical energy conversion.

Voltaic Pile Principle

The first electric battery, it produces a direct current by stacking pairs of dissimilar metal discs (e.g., zinc and copper) separated by brine-soaked cloth. Each pair forms a galvanic cell, and stacking them in series increases the overall voltage. This arrangement demonstrated the first continuous conversion of chemical energy into electrical energy, paving the way for modern electrochemistry.

Voltaic pile demonstrating electromotive force in series connection with zinc and copper cells.

Electromotive Force and Series Connection

The Voltaic pile established the principle of adding voltages by connecting electrochemical cells in series. Each zinc-copper pair, or cell, generates a characteristic electromotive force (EMF) of approximately 0.76 volts. By physically stacking these cells, Volta demonstrated that the total voltage across the pile is the sum of the individual EMFs of each cell, as described by \(V_{total} = n \times V_{cell}\).

Fresnel Zone

A Fresnel zone is one of a series of confocal prolate ellipsoidal regions in space between a transmitter and a receiver. The primary zone's boundary is the ellipsoid where the path length from transmitter to a point on the surface to the receiver is half a wavelength longer than the direct path, causing a 180-degree phase shift.

Pressure cooker and aerosol can illustrating Gay-Lussac's Law in thermodynamics.

Gay-Lussac’s Law (Pressure-Temperature Law)

Also known as Amontons's law, this principle states that for a fixed mass of an ideal gas at a constant volume, its pressure is directly proportional to its absolute temperature. The relationship is expressed mathematically as \(P \propto T\) or as a comparison between two states, \(\frac{P_1}{T_1} = \frac{P_2}{T_2}\). This describes gas behavior under isochoric (constant volume) conditions.

Gas mixing apparatus in a vintage laboratory for thermodynamics applications.

Dalton’s Law of Partial Pressures

Dalton's Law states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. The partial pressure of a gas is the pressure it would exert if it occupied the entire volume alone at the same temperature. The formula is \(P_{text{total}} = \sum_{i=1}^{n} p_i\).

Electrode polarization in a Voltaic pile experiment demonstrating hydrogen gas formation.

Electrode Polarization Limitation

A primary flaw of the Voltaic pile is electrode polarization. During operation, hydrogen gas produced at the copper cathode forms an insulating layer of bubbles on its surface. This layer increases the battery's internal resistance and reduces the surface area available for reaction. As a result, the voltage and current output of the pile drop significantly shortly after it begins to be used.